define ENTER  $0d;   for the ENTER key define BACKSPACE $08; for the BACKSPACE key define RIGHT  $81;   for the RIGHT Key define LEFT  $83;    for the LEFT key  define ARG1   $15;   for the first number of user's input  define ARG2   $16;   for the second number of user's input  define  SCREEN  $ffed; for getting the screen size define  PLOT    $fff0; for getting or setting the cursor coordinates define  SCINIT  $ff81; for initializing or clearing the screen define  CHRIN  $ffcf; for taking the input character from keyboard define  CHROUT  $ffd2; for displaying the output character to screen  jsr SCINIT  autoRestart:       ldy #$00      jsr firstArgInput ; first number input question      jsr getArg; receive the first number      jsr firstArgStore  ; store the input number       ldy #$00      jsr secondArgInput  ; second number input question      jsr getArg; receive the second number      jsr secondArgStore  ; store the input number       ldy #$00      jsr resultStr  ; for displaying the result string      jsr result ; for displaying the calculation result      jmp autoRestart  ; go back to the first step   getArg:      sec      jsr PLOT      ldx #$15      clc      jsr PLOT  numLoop:      sec      jsr PLOT      jsr CHRIN  keyboardCheck:       cmp #RIGHT ; using RIGHT arrow key, the program goes to the first digit      beq right       cmp #LEFT ; using LEFT arrow key, the program goes to the second digit      beq left       cmp #BACKSPACE ; after using BACKSPACE, one digit will be deleted      beq delete       cmp #ENTER ; after pressing ENTER, the program goes to the next process      beq move  print:      cmp #$30      bcc numLoop       clc      cmp #$3a      bcs numLoop       jsr CHROUT       sec      jsr PLOT      cpx #$17      bne numLoop      dex      clc      jsr PLOT      jmp numLoop  left:       cpx #$15 ; first digit      beq numLoop      jsr CHROUT      jmp numLoop  right:       cpx #$16 ; second digit      beq numLoop      jsr CHROUT      jmp numLoop  delete:      cpx #$15      beq numLoop      jsr CHROUT      jmp numLoop   move:      sec      jsr PLOT      ldx #$15 ; first digit      clc      jsr PLOT      sec      jsr PLOT       clc      sbc #$2F ; to calculate it, it should be subtracted by #$2f       asl      asl      asl      asl       pha       ldx #$16      clc      jsr PLOT      sec      jsr PLOT       clc      sbc #$2F ; to calculate it, it should be subtracted by #$2f      pha       ldx #$00      iny      clc      jsr PLOT      sec      jsr PLOT       pla      tax      pla       rts  firstArgStore:      sta ARG1      txa      eor ARG1      sta ARG1      rts   secondArgStore:      sta ARG2      txa      eor ARG2      sta ARG2      rts   calculateAdd:      lsr      lsr      lsr      lsr      clc      adc #$30;      jsr CHROUT       pla      and #$0F      clc      adc #$30;      jsr CHROUT       sec      jsr PLOT      ldx #$00      iny      clc      jsr PLOT       rts  result:      sec      jsr PLOT      ldx #$15      clc      jsr PLOT      sec      jsr PLOT       sed      lda ARG1      adc ARG2      cld      pha       bcc calculateAdd      ldx #$14      clc      jsr PLOT      sec      jsr PLOT      lda #$31      jsr CHROUT  firstArg: dcb "E","N","T","E","R",32,"F","I","R","S","T",32,"N","U","M","B","E","R",":",32,32,"0","0" dcb 00   secondArg: dcb "E","N","T","E","R",32,"S","E","C","O","N","D",32,"N","U","M","B","E","R",":",32,"0","0" dcb 00  results: dcb "R","E","S","U","L","T",":" dcb 00   firstArgInput:  lda firstArg,y         beq goback_main         jsr CHROUT         iny         bne firstArgInput  secondArgInput:  lda secondArg,y         beq goback_main         jsr CHROUT         iny         bne secondArgInput  resultStr:  lda results,y         beq goback_main         jsr CHROUT         iny         bne resultStr  goback_main:      rts